This invention relates to apparatus for and a method of controlling a refrigeration system. More specifically, this invention is concerned with a refrigeration system having a plurality of evaporators which are supplied high pressure refreigerant from a single source. Each evaporator is independently controlled by an expansion valve, such as a solenoid operated valve, and a control system responsive to a parameter of the refrigeration system for controlling operation of the solenoid valve thereby to regulate the flow of refreigerant through the respective expansion valve. Even more specifically, this invention relates to a solenoid valve which is periodically energized and de-energized (i.e., opened and closed) during each period of operation of the valve and a control system responsive to a refrigeration system parameter (e.g., superheat) which varies the ratio of the energization time to the de-energization time of the solenoid valve during each period of operation of the valve so as to regulate the flow of refreigerant through the valve. Still further, this system has a temperature sensor (e.g., a thermostat) within the refrigerated space served by a specified evaporator which senses the temperature within the refrigerated space and overrides the control system to block the flow of refreigerant when the temperature of the refrigerated space is below a specified level.
Typically, a refrigeration system includes a compressor, a condenser coil, an expansion device, and an evaporator coil. Refrigerant vapor is compressed to high pressure by the compressor and is directed into the condenser where the high pressure refreigerant vapor is condensed to a high pressure liquid. Many refrigeration systems such as in multiple refrigerated cabinets in a supermarket or in a multiple evaporator air conditioning system, have a plurality of evaporators supplied with refreigerant from a common condenser. An expansion device is provided between the condenser and each of the evaporators so that liquid refreigerant from the condenser may be adiabatically expanded before it enters the evaporator. In the evaporator, the low pressure refreigerant absorbs heat from the surroundings and is at least in part transformed into a vapor which is returned via a suction line to the inlet of the compressor.
In many conventional refrigeration systems, the expansion device is a so-called thermostatic expansion valve. Typically, a thermostatic expansion valve has an expansion port therein and a valve member for regulating the flow of refreigerant through the expansion port. A spring biases the valve member toward its closed position. A diaphragm actuator is provided. One side of the diaphragm is exposed to suction gas pressure while the other side is connected via a capillary tube to a thermostatic bulb in heat transfer relation with the refreigerant vapor (referred to as the suction gas) exhausted from the evaporator. The bulb is charged with a suitable volatile fluid (e.g., a refreigerant) and thus exerts a pressure force on the valve member via the diaphragm actuator counteracting the force of the spring and the suction gas pressure. Upon the thermostatic bulb's sensing an increase in temperature of the suction gas with respect to its' pressure, the net pressure force exerted on the diaphragm actuator is correspondingly increased thereby to further open the valve and to permit more refreigerant to flow through the evaporator thus resulting in a lowering of the suction gas temperature. Upon sensing a decrease in suction gas temperature, the thermostatic bulb will decrease the pressure force exerted on the diaphragm actuator and thus will permit the spring to at least partially close the valve thus lowering the flow of refreigerant into the evaporator and, in turn, raising the temperature of the suction gas.
Generally, a thermostatic expansion valve is adjusted or set to maintain the suction gas at a predetermined superheat level or setting. Superheat is a term of art which is defined as the temperature of the refreigerant vapor above the evaporated temperature of the refreigerant at the specified pressure of the refreigerant. In many refrigeration systems, the thermostatic expansion valve is preset at the factory so as to maintain a predetermined superheat level and it is impossible or impractical to vary the superheat setting of typical thermostatic expansion valves during operation of the refrigeration system so as to control the flow of refreigerant through the evaporator in response to changes in the operating conditions of the refrigeration system thereby to maximize the operating efficiency of the refrigeration system.
In an effort to overcome the shortcomings of prior thermostatic expansion valves in which the superheat setting could not be varied during operation of the refrigeration system in response to changing operating conditions (e.g., changes in the refrigeration heat load or in the outside ambient air temperature), so-called electrically operated, modulating expansion valves were developed. Such electrically operated modulating expansion valves are disclosed in the coassigned U.S. Pat. No. 3,967,781. In this electrically operated expansion valve, a plurality of bi-metallic elements and heating elements are interleafed so that upon energization of the heating elements, the bi-metallic elements expand in axial direction thereby to open the valve. Upon de-energization of the heaters, the bi-metallic metals cool and contract in axial direction thus effecting closing of the valve. This electrical heater/bi-metal actuator is often times referred to as a heat motor. By controlling the heat generated by the heaters in the heat motor, this electrically operated expansion valve could be regulated in response to system parameters (e.g., superheat) so as to regulate the flow of refreigerant through the valve.
While these prior electrically operated expansion valves worked well for their intended purpose, they had certain drawbacks in that they did require the above-mentioned heat motor actuator which was expensive. Thus, there has been a longstanding need for a low-cost, electrically operated expansion valve which has a fast response time and which is operable to regulate refreigerant flow in proportion to the requirements of the refrigeration system.
As heretofore mentioned, in many refrigeration systems having multiple evaporator coils supplied with high pressure, liquid refreigerant from a common compressor and condenser, it is advantageous to not only independently control or modulate the flow of refreigerant through each of the evaporator coils in response to a parameter associated with each of the coils, but it is also desirable to completely block the flow of refreigerant through a selected evaporator coil in the event the refrigerated space (i.e., the interior of a refrigerated cabinet or the interior of a room) cooled or refrigerated by the respective evaporator coil reaches a predetermined temperature level. However, it is also desirable that the refreigerant be permitted to flow to others of the plurality of evaporator coils supplied refreigerant by the common condenser. Heretofore, in addition to the expansion device, it was necessary to provide an independently operable solenoid valve to selectively block the flow of refreigerant to each evaporator coil. The requirement of an additional solenoid valve and a control for each solenoid valve added considerably to the complexity and cost of these systems.